TEMPO is a spaceborne instrument
mission which was selected in November 2012 through NASA’s first
EVI (Earth Venture Instrument) solicitation. NASA's EVI is part of the
agency’s ESSP (Earth System Science Pathfinder) program. The goal
of the TEMPO mission is to monitor major air pollutants from
geostationary orbit across the North American continent hourly during
daytime. The competitively selected TEMPO proposal is led by Kelly
Chance of Harvard-Smithsonian/CfA (Center for Astrophysics), Cambridge,
MA, USA. The instrument will be developed at BATC (Ball Aerospace and
Technologies Corporation), the project management is provided by
NASA/LaRC. Other collaborative institutions are: NASA/GSFC, NOAA, EPA,
NCAR, Harvard, UC Berkeley, St. Louis University, University of
Alabama, Huntsville, and the University of Nebraska. International
collaboration is provided by KARI and NIER (Korea), ESA (European Space
Agency), and Canada. 1)2)3)4)

As PI (Principal Investigator),
Kelly Chance SAO (Smithsonian Astrophysical Observatory) is responsible
for developing an instrument that will measure major air pollutants
over Greater North America, from Mexico City to the Canadian tar sands,
and from the Atlantic to the Pacific, every daylight hour. The
measurements will be taken from geostationary orbit, which will enable
continuous data collection over this region. A unique aspect of this
mission is that the instrument will be a hosted payload flown on a commercial geostationary satellite. TEMPO will be the North American geostationary component of an international constellation for air quality monitoring. 5)6)7)

The TEMPO mission builds on the
science team’s experience with the European GOME (Global Ozone
Monitoring Experiment) and SCIAMACHY (Scanning Imaging Absorption
Spectrometer for Atmospheric Cartography) missions and with the OMI
(Ozone Monitoring Instrument) flying on NASA’s Aura spacecraft.
All of these missions measure atmospheric pollution from a
sun-synchronous polar orbit. If the projected 2018-2019 launch
timeframe holds for TEMPO, its observations should coincide with
measurements from Europe’s Sentinel-4 mission, planned for launch
in 2019, and Korea’s GeoKOMPSAT-2B (Geostationary Korea
Multi-Purpose Satellite-2B), planned for launch in 2019. The Sentinel-4
mission will consist of an UVNS (Ultraviolet-Visible-Near-Infrared
Spectrometer), a sounder accommodated on MTG-S (Meteosat Third
Generation Sounder) platforms of EUMETSAT. All three missions will have
similar geostationary orbits and similar air quality observation
objectives. The three satellites will comprise a constellation for
observing continental air quality and estimating transcontinental
transport of pollution across the Atlantic and Pacific oceans. 8)9)10)11)

Instrument

Detectors

Spectral coverage

Spectral resolution

Ground pixel size, km

Global coverage

GOME (1995-2011)

Linear arrays

240-790 nm

0.2-0.4 nm

40 x 320 (40 x 80 zoom)

3 days

SCIAMACHY
(2002-2012)

Linear arrays

240-2380 nm

0.2-1.5 nm

30 x 30/60/90
30 x 120/240
(depending on product)

6 days

OMI (2004)

2D CCD

270-500 nm

0.42-0.63 nm

15 x 30 - 42 x 162
(depending on swath position)

daily

GOME-2a,b (2006, 2012)

Linear arrays

240.790 nm

0.24-0.53 nm

40 x 40
(40 x 80 wide swath;
40 x 10 zoom)

near-daily

OMPS-1 (2011)

2D CCDs

250-380 nm

0.42-1.0 nm

50 x 50 250 x 250
(depending on product)

daily

Table 1: Overview of sun-synchronous nadir heritage instruments

The NRC 2007 Decadal Survey
“Earth Science and Applications from Space: National Imperatives
for the Next Decade and Beyond” recommended missions including
GEO-CAPE (Geostationary Coastal and Air Pollution Events). The
atmosphere part of GEO-CAPE is recommended to include the measurements
proposed here .

TEMPO makes all the requisite GEO-CAPE UV/visible measurements, including O3, NO2, H2CO, and aerosols (plus several additional species including SO2).
Together with the proposed IR instrument for CO measurements, all
GEO-CAPE atmospheric requirements for North America are fulfilled.

Table 2: Alignment with 2007 Decadal Survey

Science objectives:

The TEMPO science objectives result
from many years of experience with requirements developed by the air
quality community, using observations of pollution from
sun-synchronous, polar orbits. TEMPO’s advanced capabilities over
heritage instruments are designed to answer the following science
questions:

1) What are the temporal and spatial variations of emissions of gases and aerosols important for air quality and climate?

2) How do physical, chemical, and dynamical processes
determine tropospheric composition and air quality over spatial scales
ranging from urban to continental, and temporally from diurnal to
seasonal?

3) How does air pollution drive climate forcing, and how does climate change affect air quality on a continental scale?

4) How can observations from space improve air qualityforecasts and assessments for societal benefit?

Each of these questions has been
explored from polar orbit using data from OMI onboard Aura of NASA,
SCIAMACHY on the Envisat mission of ESA, and the GOME instruments flown
on EUMETSAT missions. These instruments have surveyed key atmospheric
constituents that relate to air pollution and quality and include
tropospheric and stratospheric ozone (O3), which in the troposphere is a pollutant and a greenhouse gas; sulfur dioxide (SO2); formaldehyde (H2CO); nitrogen dioxide (NO2); glyoxal (C2H2O2);
water vapor; cloud properties; aerosol characteristics, including AOD
(Aerosol Optical Depth); and UV-B radiation. TEMPO will also measure
the same atmospheric constituents, but from geostationary orbit,
thereby allowing better spatial and temporal resolutions. 12)13)

The heritage satellite data have
revealed how air quality changes from day-to-day and year-to-year. They
have shown improvements in air quality over North America because of
regulation of power plant and automobile emissions, and also have
tracked recent severe pollution events originating over urban locations
from Asia to North America. These observations have more recently shown
the degradation of air quality with high amounts of pollution over the
Canadian tar-sand oil excavation fields. An example of NO2 data collected from OMI over the course of a year is shown in Figure 1. These data show urban and industrial hot spots that typically result from auto emissions and power plants.

TEMPO will observe
the components of pollution and their source gases over all major
cities and industrial areas in Greater North America. EPA
(Environmental Protection Agency) has designated O3, SO2, NO2,
and aerosols as criteria pollutants, and are recognized to be harmful
to health and the environment and cause property damage. Major proxies
for air pollution include formaldehyde and glyoxal in the atmosphere,
indicating the presence of non-methane volatile organic compounds
(NMVOC) emissions. The short lifetime of NMVOCs make them ideal for
locating the source of emissions from natural and anthropogenic
processes, including biomass burning. Figure 2 illustrates the TEMPO instrument’s FOR for a one-hour measurement cycle.

Legend to Figure 2:
TEMPO’s FOR is outlined in green over GNA (Greater North
America). The spread with increasing latitude is due to the projection
of the FOR as seen from a geostationary orbit where the satellite is
over the equator. The narrow white band is an exaggeration of
TEMPO’s field of view,nominally 4.7 km, which scans from east to
west over the course of an hour. The coverage from south to north will
include the range from Mexico City to the Canadian tar sands.

TEMPO is a low risk mission with significant space heritage:

• All proposed TEMPO
measurements have been made from LEO (Low Earth Orbit) satellite
instruments to the required precisions

An example of the TEMPO temporal capability is shown in Figure 3. The figure depicts a CMAQ (Community Multi-scale Air Quality) model calculation of column amounts of NO2
over the course of two days based on emissions, photochemistry, and the
local meteorology. Two observations from OMI over the same location are
also indicated, illustrating the limitations of a polar-orbiting
satellite for observing evolving time-of-day processes. The TEMPO
observational period shown will be similar to the model calculations,
but limited to daytime and cloud-free scenes. TEMPO’s
near-continuous observations will be superior to polar orbiting
satellite data for verifying model predictions and likely observe
features not seen in the model. It is also anticipated that data in the
boundary layer will be significantly improved with TEMPO’s higher
spatial resolution, which is made possible by longer integration times
from geostationary orbit.

Mission organization:
TEMPO consists of the Instrument Project, competitively selected by
NASA from the SAO proposal, and the Mission Project, directed to the
NASA/LaRC (Langley Research Center). The SAO Principal Investigator,
Kelly Chance, has delegated Project Management, Systems Engineering,
Safety and Mission Assurance, and management of the BATC prime contract
to LaRC, led by Project Manager Wendy Pennington. The Mission Project
provides the spacecraft, a commercial geostationary communications
satellite host, integration, launch, and provision of telemetry data to
the SAO. The host selection will occur once the instrument is completed
in 2017. The TEMPO Ground System, including the Instrument Operations
Center and the Science Data Processing Center are at the SAO. Data
distribution will include the RSIG (Remote Sensing Information Gateway)
of EPA (Environmental Protection Agency).

Strategic communication, public engagement, and student collaboration:
TEMPO includes a program of activities led by the SAO, and a
synergistic effort at LaRC that involves students from Minority Serving
Institutions in pursuing TEMPO-related research:

• Enhance interest in and
public awareness of NASA’s efforts to measure the distribution
and temporal variation of air pollution across North America, and,
specifically, the story of the TEMPO mission and its components
(instrument, technology, team).

• Promote science literacy by
using the TEMPO story to communicate the links between basic chemistry,
physics, and geoscience concepts, and issues of human health and
well-being.

• Engage a diverse network of
students and universities in contributing to TEMPO experimental
validation and NASA air quality research.

The activities planned to accomplish these goals include the public-oriented website http://tempo.si.edu/
; news and social media activities; the development of a mobile app and
user-friendly TEMPO data interfaces (e.g., RSIG) to enable citizen
science and broader general use of pollution data; collaborations with
STEM (Science Technology Engineering Mathematics) education partners
such as the Smithsonian Institution, the GLOBE project and My NASA
DATA; public engagement events and programs at museums, anchored by a
network of ozone gardens; and summer internships and research
experiences for students and educators at LaRC.

Development status:

• July 23, 2019: NASA has
secured a host satellite provider and ride into space for an instrument
that will dramatically advance our understanding of air quality over
North America. 15)

- Maxar Technologies of Westminster,
Colorado, will provide satellite integration, launch and data
transmission services for NASA’s Tropospheric Emissions:
Monitoring of Pollution (TEMPO),
an Earth science instrument that will observe air pollution over North
America in unprecedented detail from a geostationary orbit.

- A contract with Maxar was awarded by the U.S. Air Force Space and Missile Systems Center through its Hosted Payload Solutions contract,
a procurement mechanism that provides a pool of qualified vendors that
meet the government's needs for various hosted payload space missions
at a cost savings to the government.

- Scheduled to fly in 2022 on a
1300-class commercial satellite provided by Maxar, TEMPO will make
hourly measurements of atmospheric gases – including ozone,
nitrogen dioxide and formaldehyde as well as aerosols – across
North America, from a geostationary vantage point 35,786 km above
Earth’s equator.

- While ozone is a major protector
of life on Earth and filters out harmful ultraviolet radiation, it is
also a greenhouse gas and air pollutant. TEMPO’s new stream of
data will provide near-real-time air quality products that will be made
publicly available and will help improve air quality forecasting. TEMPO
will also enable researchers to improve pollution emission inventories,
monitor population exposure, and evaluate effective emission-control
strategies.

- The TEMPO instrument project is
led by Principal Investigator Kelly Chance, from the Smithsonian
Astrophysical Observatory (SAO) in Cambridge, Massachusetts. The
instrument was developed by Ball Aerospace in Boulder, Colorado, and is
in storage awaiting shipment to Maxar’s satellite manufacturing
facility in Palo Alto, California.

Figure 4: NASA's TEMPO instrument
will provide near-real-time North American air quality information that
will be available to the public (image credit: Ball Aerospace)

- “With the TEMPO instrument
fully spaceflight qualified and safely delivered, we are excited about
this important step and look forward to working closely with Maxar for
the successful deployment of TEMPO,” said Stephen Hall, TEMPO
project manager at NASA's Langley Research Center in Hampton, Virginia.

- TEMPO will contribute to a global
air-quality monitoring constellation that will include similar
satellites: the European Space Agency’s Sentinel-4, currently in
development, and South Korea’s GEMS (Geostationary Environment
Monitoring Spectrometer) on Geo-KOMPSAT-2, scheduled to launch in early
2020.

- The
instrument’s international science team includes partners in
North America, Asia and Europe, and is led by Chance and Deputy
Principal Investigator Xiong Liu, also from SAO. Scientists with the
Environmental Protection Agency and the National Oceanic and
Atmospheric Administration play key roles in the TEMPO science team.

• December 8, 2018: Ball
Aerospace has delivered the TEMPO spectrometer to NASA after a
successful final acceptance review. Once launched, TEMPO will be a
spaceborne ultraviolet/visible light air quality spectrometer in
geostationary orbit over greater North America. 16)

- The TEMPO instrument will provide
hourly daylight measurements of ozone, nitrogen dioxide and other key
elements of air pollution across North America, from Mexico City to
Canada and from coast-to-coast. TEMPO's high resolution will allow
pollution tracking at micro urban scales every hour and is expected to
improve air quality prediction accuracy by 50 percent.

- Ball is a part of a team that has
extensive experience in measuring the components of air quality. Ball
built the TEMPO instrument at the company's Boulder, Colorado, facility
in tandem with GEMS (Geostationary Environmental Monitoring
Spectrometer) in order to capture design efficiencies between the two
instruments, which share the same technology. GEMS is a joint
development effort by Ball and KARI (Korea Aerospace Research
Institute), and is part of a global air quality monitoring
constellation that includes TEMPO. The TEMPO team includes the
Smithsonian Astrophysical Observatory; NASA's Langley Research Center;
NASA's Goddard Space Flight Center; the U.S. Environmental Protection
Agency; and several U.S. universities and research organizations.

- NASA and Principal Investigator
Kelly Chance from the Smithsonian Astrophysical Observatory (SAO) in
Cambridge, Massachusetts, partnered with Ball to design, manufacture
and test this Earth science instrument. The rigorous environmental test
campaign conducted by Ball verifies the Tropospheric Emissions:
Monitoring of Pollution, or TEMPO, instrument will survive all of the
challenges related to launch, as well as sustained operations in
geostationary orbit. 17)

- “TEMPO exploits 30 years of
our development of ultraviolet and visible atmospheric spectroscopy to
make air quality measurements at revolutionary spectral and spatial
scales,” said Chance.

- “With completion of the
instrument and full spaceflight qualification by Ball, we are extremely
excited about the acceptance of TEMPO, which will lay the framework for
NASA and SAO’s critical air quality measurements,” said
TEMPO Project Manager Stephen Hall at NASA's Langley Research Center in
Hampton, Virginia.

- While NASA has taken ownership of
the TEMPO flight hardware, the instrument will remain in storage at
Ball until a host spacecraft is selected. NASA will partner with the
U.S. Air Force's Space and Missile Systems Center in El Segundo,
California, to employ their Hosted Payload Solutions (HoPS) contract to
issue a request for proposals from commercial companies to provide
satellite integration, launch services and ground operations for TEMPO.
Flying on this commercial spacecraft, TEMPO will make observations from
a geostationary vantage point, about 22,000 miles above Earth’s
equator.

Figure 6: Once in orbit, TEMPO
will be the first space-based instrument to monitor major air
pollutants across the North American continent hourly during daytime
(image credit: SAO)

• September 21, 2017: The TEMPO
instrument, designed and built by Ball Aerospace for NASA, has
completed spectrometer testing and verification. "The completion of
characterization and calibration of the TEMPO spectrometer is a
critical milestone in the development of our fundamental atmospheric
pollution mission, which is a result of collaboration among Ball
Aerospace, NASA's Langley Research Center, and the Smithsonian
Astrophysical Observatory" said Kelly Chance, TEMPO Principal
Investigator, Smithsonian Astrophysical Observatory. "Together, we are
all looking forward to the next steps on the way to providing hourly
atmospheric pollution measurements for greater North America." 18)

- TEMPO will
leverage the Air Force's HoPS (Hosted Payloads Solutions) contract, a
service the Air Force uses to match payloads with commercial hosts,
such as communications satellites that will fly in geostationary
orbits. NASA Langley, located in Hampton, Virginia, manages the TEMPO
Mission and will procure the spacecraft host. The launch date is yet to
be determined.

• May 4, 2016: TEMPO has now
passed the CDRs (Critical Design Reviews) for the instrument (June
24-25 at Ball Aerospace & Technologies Corp. in Boulder, CO) and
the Ground System (May 3-4 at SAO in Cambridge, MA). The instrument is
well on the way to being fabricated and characterized by Ball, with an
expected completion and delivery date in mid calendar year 2017. The
Ground System, consisting of the Instrument Operations Center and the
Science Data Processing Center, located at SAO, will be completed and
fully operational in time for the earliest possible launch date,
November 2018. 19)

• Dec. 2015: Subsystems are
completing integration and test in preparation for delivery to the
TEMPO AI&T (Assembly, Integration and Test) Team. Flight software
was completed in October 2015 and flight electronics are in final
production. Telescope and spectrometer characterization is planned for
mid-2016. Final assembly and environmental testing is planned for late
2016 with delivery to NASA/LaRC in May 2017 (Ref. 14).

• June 10, 2015: The TEMPO
instrument, developed by BATC (Ball Aerospace & Technologies
Corp.), is headed toward its CDR (Critical Design Review) this month
following an earlier milestone that included PDR (Preliminary Design
Review). Ball is now well into the fabrication of the instrument
following the successful PDR and a confirmation review. 21)

• On April 10, 2015, TEMPO was
confirmed by NASA’s Science Mission Directorate to continue into
the development phase of the project. 22)23)

Launch: A launch of TEMPO is scheduled for 2022 (Ref. 15).
NASA will arrange launch and hosting services. NASA plans to award a
Task Order under the Air Force’s SMC (Space and Missile Systems
Center) HoPS (Hosted Payload Solution) contract (the Air Force is
supporting NASA pursuant to an Acquisition Assistance agreement). The
SMC manages HoPS contracts for the USAF. 24)

The TEMPO instrument is an imaging
Offner grating spectrometer measuring solar backscattered Earth
radiance. The design will incorporate many of the features and lessons
learned from heritage spectrometers flown by Europe and the U.S. The
key instrument characteristics and capabilities are: 25)26)

TEMPO measurements will capture the
high variability in the diurnal cycle of emissions and their evolving
chemistry, which occurs mostly during the day. TEMPO’s footprint
— smaller than for previous missions measuring air quality
— will resolve pollution sources at suburban scales. With both
high temporal and spatial resolution, TEMPO data will improve emission
inventories, monitor population exposure to pollution, and make
possible effective emission control strategies by regulatory agencies.

The TEMPO instrument is a grating
spectrometer, sensitive to visible and ultraviolet wavelengths of
light, which will be attached to the Earth-facing side of a commercial
telecommunications satellite (to be selected) in geostationary orbit.
This allows TEMPO to maintain a constant view of North America so that
the instrument's light-collecting mirror can make a complete East to
West scan of the FOR (Field of Regard) each and every hour of the day.
By measuring sunlight reflected and scattered from the Earth's surface
and atmosphere back to the instrument's detectors, TEMPO's ultraviolet
and visible light sensors will provide spectra of ozone, nitrogen
dioxide, and other elements of daily atmospheric chemistry cycles. 27)

Legend to Figure 7:
The scan mirror projects the slit field of view onto GNA (Greater North
America). Multiple images are taken at each scan mirror position.
Mirror scans from East to West, imaging GNA on a hourly basis.

TEMPO will be integrated to the host
spacecraft with the nominal optical axis pointed at 36.5°N,
100°W (~5.8° from spacecraft nadir).Its field of regard is
designed to cover greater North America as seen from any GEO orbit
longitude within 80°W to 115°W. The TEMPO instrument consists
of a number of subsystems as indicated in the block diagram shown in
Figure 8. The yellow arrows indicate the
path of light from the aperture through the optical assembly to the
focal plane subsystem (Ref. 10).

Legend to Figure 8:
The TEMPO instrument consists of an off-axis reflective Schmidt
telescope and an Offner spectrometer. The yellow arrows through the
block diagram represent the optical path through the instrument. DITCE
(Differential Impedance Transducer Conditioning Electronics). A graphic
of the instrument cross section is shown along with images of actual
flight hardware produced to date (Figure 10).

Figure 10: Illustration of the TEMPO instrument and its elements (image credit: BATC)

Parameter

Value

Comment

Instrument mass

148 kg

Mature mass = CBE + 15%

Instrument volume

1.4 x 1.1 x 1.2 m (x, y, z)

Average operational power

134 W

Mature power = CBE + 10%

FOR (Field of Regard)

4.76º N/S x 8.95º E/W

Coverage of GNA (Greater North America)

GSD (Ground Sample Distance)

2.21 x 4.97 km (vertical x horizontal)

At Geodetic 36.5º N, 100º W

Spectral range

290 – 490 nm, 540-740 nm (UV, VIS)

Spectral resolution, and sampling

0.57 nm, 0.2 nm

Maximum SNR

2700 @ 330-340 nm, EOL

Spatial resolution

2.1 km x 4.5 km @ 36.5 N, 100 W

IFOV: N/S x E/W

Spectra per hour

2000 N/S x 1250 E/W

Revisit time

1 hour

Geolocation uncertainty

2.8 km

Albedo calibration uncertainty

2.0% λ-independent, 0.8% λ-dependent

Table 3: TEMPO instrument key parameters

The CMA (Calibration Mechanism
Assembly) controls the instrument aperture. It consists of a wheel
containing four selectable positions: Closed, open, working diffuser
and reference diffuser. The ground fused silica diffusers allow
recording of the top-of-atmosphere solar irradiance. Earth-view
radiance measurements are made in the open position. The working
diffuser is used on a daily basis and the reference diffuser is used to
trend any degradation of the working diffuser from radiation exposure
and contamination. Dark scene data are collected with the wheel in the
closed position (Ref. 10).

The SMA (Scan Mechanism Assembly)
steps the projected TEMPO instrument slit image,or FOV (Field of View),
aligned in the North-South direction) across the TEMPO field of regard
and compensates for unwanted spacecraft motion. When the instrument is
collecting image data, the FOV is held stable at a given ground
location while individual images are recorded before stepping to the
next location. The SMA is housed as the first optic within the
telescope optical assembly. The SMA consists of a silicon carbide
mirror gimballed to a two-axis mechanism involving two flex pivots per
axis. The mechanism is actuated inductively using a network of voice
coils and magnets. The mirror position is measured using DITs
(Differential Impedance Transducers). The SMA is close-loop controlled
using realtime attitude data supplied by the host spacecraft.

The optomechanical subsystems
consist of a telescope and a spectrometer assembly. Primarily, the
optical design employs reflective optics with simple geometries(Figure 9).
The f/3 Schmidt-form telescope consists of the T1 Scan mirror, the T2
Schmidt mirror and a T3 and T4 with final projection onto the slit of
the spectrometer assembly. The telescope mirrors are coated with
UV-enhanced aluminum, with the exception of the T2 optic, which has a
band-blocking coating to minimize straylight biases within the
spectrometer.

The Offner-type spectrometer was
chosen due to its compact design and superior re-imaging performance
and is similar to the OMPS (Ozone Mapping Profiler Suite) nadir
spectrometer. The spectrometer consists of a slit, a quartz wave plate
(for polarization mitigation), a diffraction grating, a corrector lens
and a CCD window/order sorting filter. The mechanically ruled grating
(500 lines/mm) is a convex paneled (3 partite) optic with a blaze angle
of 5º at 325 nm. The optical benches are a truss-type design
constructed of composite tubes with titanium fittings. The
optomechanical system is athermal and actively temperature-controlled
for superior spectral stability over changing diurnal and seasonal
thermal environments.

The FPA (Focal Plane Array)and FPE
(Focal Plane Electronics) comprise the focal plane subsystem. The FPA
contains two separate, but identically designed, 1k x 2k pixels,
full-frame transfer, CCD (Charge Coupled Device) detectors. There are
2k pixels each in the spatial direction (along the slit) and 1k pixels
each in the spectral direction. 290 nm to 490 nm is measured by the UV
CCD and 540 nm to 740 nm by the VIS CCD. The CCDs are back-thinned and
have an anti-reflection coating for enhanced operation in the UV. The
CCDs are read-out and digitized simultaneously to create spectra with
the same period of integration (~118 ms in duration). Multiple
integrations (~21) are added together on-board, for a single scan
mirror position, before transferring to the host spacecraft for
downlink. The CCDs are passively cooled with a dedicated thermal
connection to a cold biased spacecraft thermal interface and stabilized
with a heater on the thermal connection. The spectral regions to be
measured by TEMPO are illustrated in Figure 12 by reflectances for various scenes measured by the ESA (EuropeanSpaceAgency) GOME-1 instrument.

Figure 12:
The spectral regions to be measured by TEMPO are illustrated with
reflectance spectra for the range of surface and atmosphere scenes
using reflectances derived from ESA GOME-1 measurements. The dashed
blue boxes indicate the TEMPO spectral coverage (image credit: TEMPO
collaboration)

The TEMPO INR solution uses a combination of flight hardware and ground software, as shown in Figure 15,
to accurately assign geographic locations to pixels and assure uniform,
gapless, and efficient coverage of greater North America. TEMPO INR
relies on GOES weather satellite imagery for pointing truth. This is
available with ≤ 5 min latency with respect to realtime and is more
accurately registered to the Earth than that required by TEMPO. First,
a GOES-like image is constructed by weighting the TEMPO spectral planes
in accordance with the GOES relative spectral response function. Next,
templates are extracted from the GOES-like image and matched against
GOES imagery, creating a set of tie-point measurements in progression
as TEMPO scans across the domain.

A Kalman filter
with a high-fidelity model of the TEMPO system embedded within it is at
the heart of the TEMPO INR system. Its state vector is updated with
each tie-point measurement and propagated in between measurements using
modeled dynamics and spacecraft attitude telemetry from onboard
gyroscopes. A tracking ephemeris provided by the host spacecraft
operator is also input in to the Kalman Filter. The state vector
estimates for each TEMPO dwell time can be used to determine the Earth
locations of each of its pixels in realtime. Using attitude data to
stabilize the TEMPO line-of-sight pointing by providing control inputs
into the SMA as described above enables uniform and gapless coverage of
the domain. Other than providing ephemeris and gyroscopes, the host
spacecraft is only required to orient TEMPO towards the Earth with an
accuracy of 1100 µrad (3σ), a capability well within that
of a modern commercial communications satellite. Scan tailoring
parameters are routinely generated by the INR processing to predict
offsets in pointing to keep the TEMPO field of regard centered over a
target Earth location. They are applied to the scan starting
coordinates in the scan tables defining the data acquisition schedule,
which is updated weekly via command uploads. The tailored scan
coordinates reduce the need to overcover the domain, making science
data collection more efficient.

The tie-point paradigm is that TEMPO
and GOES are looking at the same thing, at the same time, and in the
same spectral band;therefore, knowledge of the geographic coordinates
of unknown features, even clouds, seen by GOES can be transferred to
TEMPO. However, it is important to manage the parallax that may arise
because TEMPO is not necessarily stationed at a longitude nearby a GOES
spacecraft. We do that by either using a priori knowledge of object
height (cloud top height assignment or topographic height for clear
skies) to correct the measured displacement or by binocularly solving
for the height of the unknown object by matching it with imagery from
two different GOES satellites. The Level 2 requirement is that the
angular uncertainty of a fixed point be less than 82 µrad
(3σ), 4 km in position on the ground at the center of the field
of regard. INR performance for TEMPO is figured to be generally better
than 56 µrad (3σ), 2.8 km at the center of the field of
regard.

A typical day of operations for TEMPO is shown in Figure 13.
Earth scans are collected with one-hour revisit time during daylight
and twilight (two hours before and after full sunlight). The actual
daily timeline will vary seasonally, accounting for temperature and
straylight. Solar calibrations may be made when the sun is unobscured
at angles ±30° to the instrument boresight. Dark frame
calibrations are required to support radiometric accuracy. The nominal
scan pattern consists of a series of East-West (E-W) scan mirror
steps(~1282) across the field of regard, with the image of the
spectrometer slit on the ground defining the North–South extent
of the FOV. A continuous subsection of the field of regard may be
scanned at shorter revisit time (5–10 min) for episodic pollution
events or focused studies.

Science data collection may be
optimized in the early morning and late afternoon when significant
portions of the field of regard have SZAs (Solar Zenith Angles)
>80º. Data with SZA >80° are unsuitable for most of the
planned atmospheric chemistry measurements, but can constitute 20% of
the data collected with the nominal coast-to-coast hourly scanning. The
morning optimized data collection will terminate the nominal E-W scan
pattern when the SZA >80° throughout the FOV (governed mainly by
the SZA at the southern extent of the FOV), and proceed back to the
East-most portion of the field of regard to commence a new E-W scan.
Since the entire field of regard is not scanned, the revisit time is
less than the nominal 1-h (as small as5 min) as the scan termination
point follows the terminator (SZA>80°) across greater North
America. In the afternoon, as the evening terminator progresses
westward across the field of regard, data collection will use multiple
scan tables to essentially move the initial point of the scan, skipping
FOVs with SZA >80º.

TEMPO is the first NASA EVI (Earth
Venture Instrument) and the first to go through the process of finding
a host spacecraft. The primary challenge for a hosted payload developer
is the lack of a host with whom to discuss and negotiate interface
requirements. To assist payload developers with this challenge, NASA
has developed a set of CII (Common Instrument Interfaces) guidelines.
The CII guidelines were developed by using a compilation of interface
and environmental requirements. The guidelines are helpful in that they
inform payload suppliers with the wide range of potential requirements.
For instance, the launch loads contained in the CII envelope many
potential spacecraft buses and launch vehicles. Using the loads
indicated in the CII would cause a payload developer to over-specify
and over-design a payload from a structural point of view. This
potential issue can be mitigated with an early selection of a host
spacecraft provider. In the absence of an early selection, it is
recommended that the CII guidelines be tailored based on the
probability of each spacecraft and launch vehicle combination. This
approach helps to identify low-probability combinations of spacecraft
and launch vehicle that may be driving requirements (and cost) of the
hosted payload. The TEMPO mission team at NASA/LaRC in charge of the
selection of the TEMPO host has been working closely with the TEMPO
instrument team to help minimize any risks of over-design and
over-test.

The TEMPO mission requires precise
pointing and stability for imaging. Undesirable jitter and drift can
cause degradation of the TEMPO data. In a dedicated Earth remote
sensing mission, the spacecraft and instrument would be designed such
that the system would meet these pointing stability requirements.
Commercial communication satellites have stability requirements to
carry out radio frequency (RF) communications missions. While RF
communication missions require good pointing, they can accommodate some
level of drift. These levels of drift would significantly impact an
optical remote sensing mission such as TEMPO. To mitigate this issue,
the TEMPO instrument includes a high precision scan mirror. The scan
mirror uses position feedback from an inertial sensor (3-axis
gyroscope). The scan mirror controller uses closed-loop control to take
out spacecraft jitter and drift, while scanning the instrument’s
narrow East-West FOV, over the wide-area field of regard. The TEMPO
scan mechanism assembly could provide this capability to other hosted
payloads that require similar high-precision pointing (Figure 14).

The TEMPO space segment consists of
the TEMPO instrument and the host spacecraft. The host spacecraft
vendor is responsible for the integration of the TEMPO instrument to
the host spacecraft.

The ground segment consists of the
IOC (Instrument Operations Center)and the interface to the host SOC
(Spacecraft Operations Center). The science segment includes the SDPC
(Science Data Processing Center). SAO developed the IOC and the SDPC.
The ground segment commands the instrument, monitors instrument health
and status telemetry, and to receive and transfer science data from the
instrument to the IOC and SDPC. The SDPC receives science and telemetry
data from the IOC, performs all data processing needed to generate
science products, and distributes data products including transmitting
all data and products for archival (Ref. 10).

The TEMPO ground system commands the
instrument, monitors instrument health and status, and produces Level-0
science data for delivery to the SDPC. TEMPO instrument telemetry is
downlinked to the hosts's SOC (Science Operations Center) and then
forwarded to the TEMPO IOC, where the telemetry packets are
decommutated and processed to Level 0. The IOC autonomously
limit-checks the instrument H&S (Health and Status) data and alerts
the operators of any out-of-limit conditions. The H&S data are
stored in the IOC (Instrument Operations Center) for the life of the
mission to support remote monitoring, trending, and anomaly resolution.
The web-based remote monitoring system allows the operators to
graphically view the H&S data in realtime and to create plots of
the data over arbitrary time intervals. The IOC extracts image data
from the telemetry packets to reconstruct CCD image frames.

The reconstructed images are sent to
the SDPC for assembly into granules for further processing. The IOC
also sends gyroscope and scan mechanism controller data to the SDPC to
support image pixel geolocation. The commanding component of the IOC
utilizes the Ball Aerospace COSMOS command and telemetry system in
conjunction with the instrument simulator for command planning,
generation, and validation. TEMPO operates from a 14-day command
sequence that controls the day-to-day scanning and calibration
activities. New14-day command sequences that incorporate the latest
predicted ephemeris and scan tailoring information from INR (Image
Navigation and Registration) processing are developed and uplinked on a
weekly basis. In the event of a special observation, the currently
executing command sequence can be interrupted and replaced.

Data processing and availability:

The TEMPO instrument operations
center at SAO receives telemetry from the host spacecraft operations
center. Level 0 science data are passed to the TEMPOS DPC for
processing and distribution. In the SDPC, Level 0 data from radiance
scans are gathered into granules having sufficient east-west coverage
to enable geolocation, typically about 5 min of scan data. Initial
radiometric calibration converts the Level 0 digital numbers into
physical units, including a stray light correction, and an initial
wavelength calibration. When initial radiometric calibration is
complete,INR processing derives the latitude-longitude coordinates of
the center and corners of each pixel. Knowledge of the scattering
geometry facilitates the polarization correction that is applied in
producing the final Level 1 radiance spectra.

An irradiance spectrum is acquired
each night when the sun is 30º from the boresight, usually about
two hours before midnight. Consistent irradiance measurement geometry
reduces variability associated with angular dependence of the diffuser
bidirectional transmittance distribution function. For each Level 1
radiance granule, the Level 2 cloud product is generated first, then
the other Level 2 products are generated using the cloud product as
input along with the most recent irradiance measurement. The
computational cost of the Level 2 ozone profile product is much greater
than that of the other Level 2 data products. For this reason, the
Level 2ozone profile product is generated by first coarsely binning
each radiance granule, then dividing each granule into many small
blocks (e.g. 64 blocks), and processing the small blocks in parallel.
Level 3 data products are generated by gridding hourly scans of level 2
data products to standard longitude-latitude grid cells (except the
cloud product). All TEMPO data products are stored in net CDF4/HDF-5
format in a data archive at SAO for the life of the mission.

New data products are made available
from a website at SAO where data products are organized by date and
product type. The website provides access to the most recent 30 days of
TEMPO data. Throughout the mission, new data products are regularly
(e.g. weekly) transferred to NASA's Atmospheric Science Data Center for
public distribution. To facilitate browsing and subsetting, TEMPO data
are also made available through EPA's (Environmental Protection Agency)
RSIG (Remote Sensing Information Gateway).

Global constellation and international partnerships:

TEMPO is part of a virtual satellite
constellation, fulfilling the vision of the IGOS (Integrated Global
Observing System) for a comprehensive measurement strategy for
atmospheric composition. TEMPO team members have been key participants
in international activities to define this potential under the auspices
of CEOS (Committee on Earth Observation Satellites), and as members of
Korean and European mission science teams. The constellation will
become a reality in the 2020 timeframe, including regional
geostationary observations over the Americas (TEMPO), Europe
(Sentinel-4), and Asia (GEMS) combined with LEO (Sentinel-5/-5P)
observations to provide full global context. Mission team members are
now working together on data harmonization activities, featuring common
data standards and validation strategies, to provide truly
interoperable data products from this satellite constellation.

Europe: S4 (Sentinel-4)

The S4 mission, expected to launch
in 2021, together with Sentinel-5 and the Sentinel-5 Precursor
missions, is part of the Copernicus Space Component dedicated to
atmospheric composition. The objective of the S4 mission is to provide
hourly tropospheric composition data mainly on an operational basis in
support of the air quality applications of the Copernicus Atmosphere
Monitoring Services over Europe.

The S4 instrument is a UV/VIS/NIR
spectrometer (S4/UVN) that will fly on the geostationary Meteosat Third
Generation-Sounder (MTG-S) platforms in order to measure Earth radiance
and solar irradiance. The S4/UVN instrument measures from 305 nm to 500
nm with a spectral resolution of 0.5 nm, and from 750 nm to 775 nm with
a spectral resolution of 0.12 nm, in combination with low polarization
sensitivity and high radiometric accuracy. The instrument observes
Europe with a revisit time of one hour. The spatial sampling distance
varies across the geographic coverage area and is 8 km at the reference
location at 45°N.

ESA is responsible for the
development of the S4/UVN instrument, the Level-1b Prototype Processor
(L1bPP), and the Level-2 Operational Processor (L2OP). Instrument and
L1bPP are built by a consortium led by Airbus Defence and Space. The
L2OP is developed by a consortium led by DLR. It covers key air quality
parameters including tropospheric amounts of NO2, O3, SO2, H2CO, and C2H2O2,
as well as aerosols, clouds, and surface parameters. Two S4/UVN
instruments are expected to be flown in sequence spanning an expected
mission lifetime of 15 years. EUMETSAT operates the instrument and
processes the mission data up to Level-2.

Korea: GEMS (Geostationary Environment Monitoring Spectrometer)

GEMS is a scanning UV/VIS imaging
spectrometer planned for launch into geostationary orbit in 2019 over
Asia to measure tropospheric column amounts of O3, NO2, H2CO, SO2 and
aerosols at high temporal and spatial resolution. With the recent
developments in remote sensing with UV/VIS spectrometers, vertical
profiles of O3 (e.g. and centroid height of aerosols(e.g.
can be retrieved as well. The required precisions of the products are
comparable to those of TEMPO and S4. GEMS is a step-and-stare scanning
UV/VIS imaging spectrometer, with a scanning Schmidt telescope and
Offner spectrometer,similar but not identical to TEMPO. The spectral
coverage of GEMS is from 300 nm to 500 nm, with the resolution of 0.6
nm, sampled at 0.2 nm. A UV-enhanced 2D CCD takes images, with one axis
spectral and the other north-south (NS) spatial, scanning from east to
west (EW) over time. GEMS covers important regions in Asia including
Seoul, Beijing, Shanghai, and Tokyo from 5ºS to 45ºN in
latitude, and from 75ºE to 145ºE in longitude, with the
spatial resolution of 7 km (NS) x 8 km (EW) at Seoul. The planned
minimum mission lifetime is 7 years.

On orbit calibrations are planned,
making daily solar measurements and weekly LED light source linearity
checks. For the solar calibration, there are two transmissive
diffusers, a daily working one and a reference diffuser used twice a
year to check the degradation of the working one. Dark current
measurements are planned twice a day, before and after the daytime
imaging. In order to avoid dark current issues and RTS (Random
Telegraph Signal), the CCD is cooled to -20°C. Spectral stability
is required to be better than 0.02 nm over daily observation hours,
straylight less than 2%, polarization sensitivity less than 2% at the
instrument level, and the instrument system level MTF (Modulation
Transfer Function) better than 0.3 Nyquist.

Canada:

TEMPO provides a unique opportunity
to provide consistent and timely air quality information to over 99.5%
of the Canadian population. TEMPO data are of particular interest to
Canada given the challenges in observing its vast land area from
ground-based measurements alone. A primary application of TEMPO data is
assimilation into the EC (Environment Canada) air quality forecast
system for the purpose of improving air quality forecasts over a domain
that largely overlaps with the TEMPO field of regard. Other priority
application areas include environmental assessment, epidemiological
analyses, health impact studies, and monitoring of natural disasters
such as forest fires.

To fully exploit TEMPO, Canada is
interested in enhancing TEMPO data quality at higher latitudes
where:larger average solar and viewing angles lead to reduced
sensitivity of some gases (e.g.,O3 and NO2) to the boundary layer; stratospheric abundances of some absorbers (also O3 and NO2)
are larger and display greater variability thereby making them more
difficult to remove; TEMPO pixel sizes are larger than at lower
latitudes; and where trace gas retrievals in forest fires are
complicated by high aerosol loading. Issues in the representation of
snow-covered surfaces also lead to larger uncertainties. Canadian
academia and government are collaborating to address these by
developing direct inversions to improve sensitivity in the boundary
layer, developing methods to better constrain stratospheric abundances
including assimilation of stratospheric profiles, implementing an
improved representation of snow in the inversions, and developing
algorithms to explicitly account for th e effects of aerosols on trace
gas retrievals. Validation of TEMPO observations over Canada is also a
priority with an expansion of the Canadian Pandora network and an
aircraft measurement campaign being planned.

Mexico:

The TEMPO field of regard will cover
at least 78% of the Mexican territory, including the Mexico City
Metropolitan Area with its 21 million inhabitants . Mexico City is one
of the best-monitored urban areas in the world with an air quality
network (GDF-SEDENA) run by the city government that has records
beginning in 1986 and is now comprised of over 30 stations. Most of
these stations measure air pollutants (e.g., O3, CO, NOx, SO2,
PM2.5) continuously and are used to advise the authorities when
additional measures need to be taken in case of critical pollution
events and research purposes. Other large cities with air quality
measurements include Monterrey, Guadalajara, Toluca, Tijuana and
Mexicali but most of the country lacks information to assess the
impacts of air pollution. Therefore, the national environmental
institute INCECC (Instituto Nacional de Ecologíay Cambio
Climático) and SAO have signed a memorandum of understanding in
order to work towards making the data produced by TEMPO available and
useful for the Mexican public.

A strong academic collaboration has
been established with UNAM (National Autonomous University of Mexico).
Together with other institutions and universities, a nationwide network
of atmospheric observatories (RUOA) has been established to
continuously measure additional species including black carbon and
greenhouse gases. A network of four MAX-DOAS instruments has been
installed in Mexico City, producing NO2 and H2CO
total vertical columns with high temporal resolution. The data produced
in Mexico City and other locations will be part of the validation
efforts of TEMPO.

The TEMPO observations over Mexico
are of particular interest to better characterize emissions from
industrial and urban regions which are poorly studied. Biomass-burning
sources particularly during the distinctive dry season and the harmful
agricultural practices in many parts of the country are monitored by
TEMPO. This will be of great value to alert vulnerable communities and
prevent damages as well as to increase the understanding of the
variability and dynamics of transported pollution plumes.

TEMPO science products:

Standard data products: TEMPO will measure as standard data products the quantities listed in Table 4 for greater North America. The O3 products, NO2, and H2CO
are required products and meet precision requirements up to 70° SZA
(Solar Zenith Angle). The spatial and temporal resolutions and SZA
constraints are for meeting the requirements only. Operational
retrievals will be done hourly at native spatial resolution (~2.1 x 4.4
km2) during the day-lit period except for ozone profile retrievals at the required spatial resolution of ~8.4 x4.4 km2
(four co-added pixels for increased signal and reduction of
computational resources). Precisions are listed for all species for
four co-added pixels, as that is the form of the NASA precision
requirements for the mission.

Spatial resolution: 8.4 x 4.4 km2 at the center of the field of regard. Time resolution: Hourly unless noted.
Expected precision is viewing condition dependent. Results are for nominal cases.
UV indices, including the erythemally weighted irradiance are derived from O3 and other parameters.

Volcanic SO2 (column
amount and plume altitude) and diurnal out-going shortwave radiation
and cloud forcing are potential research products. Additional
cloud/aerosol products are possible using the O2-O2 collision complex and/or the O2
B band. Additional aerosol products will combine measurements from
TEMPO and GOES-R. Nighttime “city lights” products, which
represent anthropogenic activities at the same spatial resolution as
air quality products, may be produced twice per day (late evening and
early morning) as a research product. Meeting TEMPO measurement
requirements for NO2 (visible) implies the sensitivity for city lights products over the CONUS within a 2 h period at 8.4 x 4.4 km2 to 4.25 x 10-9 W cm-2 sr-1 nm-1.

International collaboration

A single geostationary satellite
views only one sector of the globe, limiting the capability to observe
sources of pollution outside the instrument FOR. Fortunately, both
Korea with GEMS (Geostationary Environment Monitoring Spectrometer) to
be flown on GeoKOMPSAT-2B, and Europe (ESA and EUMETSAT) with the UVNS
(UV NIR Spectrometer) on Sentinel-4, plan to develop and launch their
own instruments to fly on geostationary satellites to measure air
composition and quality in the 2017-2022 timeframe. These missions will
have measurement capabilities and science objectives similar to TEMPO.
Therefore, it will be possible, with a minimum of three geostationary
satellites positioned to view Europe, East Asia, and North America, to
collectively provide near-global coverage in the Northern Hemisphere.
The synergy of contemporaneous satellite missions having similar
observing capabilities and data distribution protocols will provide
unique opportunities to advance understanding of the interactions
between regional and global atmospheric composition in the troposphere.
This would include assessments — not possible before — of
emission sources, intercontinental pollution trans-port, and regional
interactions between air quality and climate. These activities would
address several societal benefit areas of GEOSS (Global Earth
Observation System of Systems), (Ref. 8).

In addition to
TEMPO, the European Sentinel-4 and the Korean GeoKOMPSAT missions have
been approved. By harmonizing these missions it is possible to improve
the scientific return and societal benefit of each of the individual
missions while beginning a global observing system that will be
impossible for any one country to implement alone. Best efforts to
cooperate on defining common requirements and data products can enable
improved designs for all instruments and allow cost savings by
minimizing duplication of effort. While recognizing that unique
requirements likely exist for individual missions, this approach
defines common objectives that build a foundation for a future
integrated observing system for atmospheric composition, as envisioned
in 2004 by the IGOS (Integrated Global Observing System).

Figure 16:
The potential global coverage of the three geostationary missions,
separated by roughly 120º in longitude (image credit: J. Ziemke,
GSFC, and the OMI and MLS instrument and algorithm teams)

Legend to Figure 16:
The 3 images show simulation of average tropospheric ozone — an
indicator of poor air quality — using data from Aura’s OMI
and MLS (Microwave Limb Sounder) when viewed from three geostationary
positions over major continents for May-July 2008. The three
geostationary missions (i.e., originated by NASA, ESA, and Korea)
however, will focus on the Northern Hemisphere only. Shades of purple
and blue correspond to 10-20 DU (Dobson Units), representing low ozone
amounts, while lighter shades correspond to 35-50 DU. Green, yellow,
and red indicate high-pollution areas.

The simultaneous development of
these individual missions to acquire data over Earth’s major
industrialized regions presents a real opportunity for international
collaboration to improve the preparation for these missions and their
combined capabilities within a global system. Best efforts are all
ready underway to cooperate on defining common measurement
requirements, retrieval algorithms, validation, data quality, and
access to achieve the above goals. Consistency of data products will
result in better understanding of the science, improved application
capabilities, and subsequent use by regulatory agencies.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).